Processes and systems for separating carbon dioxide in the production of alkanes

ABSTRACT

A method for separating CO 2  from C 2  to C 5  alkanes includes introducing a first stream including C 2  to C 5  alkanes and CO 2  into a first separation zone, the first separation zone including a hydrocarbon solvent, and separating the first stream into a recycle stream and a second stream in the first separation zone. The recycle stream including CO 2  and one or more of CO, H 2 , and CH 4 , and the second stream including C 2  to C 5  alkanes. The method further includes introducing the second stream into a second separation zone, and separating the second stream into a third stream and a fourth stream, wherein the third stream includes C 2  alkanes and the fourth stream includes C 3  to C 5  alkanes.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/491,663 filed Apr. 28, 2017, entitled PROCESSES AND SYSTEMS FORSEPARATING CARBON DIOXIDE IN THE PRODUCTION OF ALKALINES, the contentsof which are hereby incorporated by reference in its entirety.

BACKGROUND Field

The present specification generally relates to processes and systems forseparating carbon dioxide (CO₂) in the production of alkanes and, morespecifically, is directed to processes and systems that separate CO₂from a product stream comprising alkanes using a hydrocarbon solvent.

Technical Background

In various processes—such as, for example, a process for forming lightalkanes (e.g., C₂ to C₅ alkanes) using a hybridcatalyst—hydrocarbon-derived gas streams, such as, for example, syngas,are converted to light alkanes, CO₂, and methane (CH₄). In a hybridprocess, alkanes are formed by carbon monoxide hydrogenation tohydrocarbons, potentially via a methanol intermediate. The CO₂ isgenerally formed by a traditional water gas shift reaction. The productstream in these processes may also contain unreacted hydrogen (H₂) andcarbon monoxide (CO), which are desirably recycled back to the reactorthat forms the light alkanes from the hydrogen-containing gas stream toachieve a highly efficient system that does not unnecessarily waste rawmaterials.

In the above-described processes, CO₂ may be recycled back to thereactor that forms the light alkanes from the hydrogen-containing gasstream, or it is purged from the system. However, separating the CO₂from the light alkane products can be challenging. Although conventionalsystems for separating light alkanes from CO₂ exist, they can be costly,inefficient, and may utilize undesirable chemicals.

Accordingly, a need exists for processes and systems that canefficiently separate light alkane products and CO₂.

SUMMARY

According to one embodiment, a method for separating CO₂ from C₂ to C₅alkanes, comprises: introducing a first stream comprising C₂ to C₅alkanes and CO₂ into a first separation zone, the first separation zonecomprising a hydrocarbon solvent; separating the first stream into arecycle stream and a second stream in the first separation zone, whereinthe recycle stream comprises CO₂ and one or more of CO, H₂, and CH₄, andthe second stream comprises C₂ to C₅ alkanes; introducing the secondstream into a second separation zone; and separating the second streaminto a third stream and a fourth stream, wherein the third streamcomprises C₂ alkanes and the fourth stream comprises C₃ to C₅ alkanes.

In another embodiment, a system for separating CO₂ from C₂ to C₅alkanes, comprises: a first separation zone comprising a hydrocarbonsolvent and that is configured to separate a first stream comprising C₂to C₅ alkanes and CO₂ into a recycle stream and a second stream, whereinthe recycle stream comprises CO₂ and one or more of CO, H₂, and CH₄, andthe second stream comprises C₂ to C₅ alkanes; and a second separationzone that is fluidly connected to the first separation zone and that isconfigured to separate the second stream into a third stream and afourth stream, wherein the third stream comprises C₂ alkanes and thefourth stream comprises C₃ to C₅ alkanes.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from that description or recognized by practicing theembodiments described herein, including the detailed description whichfollows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description describe various embodiments and areintended to provide an overview or framework for understanding thenature and character of the claimed subject matter. The accompanyingdrawings are included to provide a further understanding of the variousembodiments, and are incorporated into and constitute a part of thisspecification. The drawings illustrate the various embodiments describedherein, and together with the description serve to explain theprinciples and operations of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a conventional system for separating CO₂ inthe production of alkanes;

FIG. 2 schematically depicts a first system for separating CO₂ in theproduction of alkanes according to one or more embodiments disclosed anddescribed herein; and

FIG. 3 schematically depicts a second system for separating CO₂ in theproduction of alkanes according to one or more embodiments disclosed anddescribed herein.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of processes andsystems for separating CO₂ in the production of alkanes. Wheneverpossible, the same reference numerals will be used throughout thedrawings to refer to the same or like parts. In one embodiment, A methodfor separating CO₂ from C₂ to C₅ alkanes includes introducing a firststream including C₂ to C₅ alkanes and CO₂ into a first separation zone,the first separation zone including a hydrocarbon solvent, andseparating the first stream into a recycle stream and a second stream inthe first separation zone. The recycle stream including CO₂ and one ormore of CO, H₂, and CH₄, and the second stream including C₂ to C₅alkanes. The method further includes introducing the second stream intoa second separation zone, and separating the second stream into a thirdstream and a fourth stream, wherein the third stream includes C₂ alkanesand the fourth stream includes C₃ to C₅ alkanes. The third streamcomprises C₂ alkanes, and the fourth stream comprises C₃ to C₅ alkanes.In another embodiment, a system for separating CO₂ from C₂ to C₅ alkanesincludes a first separation zone comprising a hydrocarbon solvent andthat is configured to separate a first stream comprising C₂ to C₅alkanes and CO₂ into a recycle stream and a second stream, and a secondseparation zone, which is fluidly connected to the first separationzone, and that is configured to separate the second stream into a thirdstream and a fourth stream. The recycle stream includes CO₂ and one ormore of CO, H₂, and CH₄, and the second stream includes C₂ to C₅alkanes. The third stream includes C₂ alkanes and the fourth streamincludes C₃ to C₅ alkanes.

As used herein, the term “light alkanes” refers to C₂ to C₅ alkanes,including, but not limited to, ethane, propane, n-butane, isobutane,pentane, isopentane, and neopentane.

The scheme used to separate and control the recycle streams, includingseparation of the CO₂ in the various streams, will impact reactorcomposition, reactor flow, CO conversion, CO₂ production or conversionacross the reactor, and reactor productivity. Conventional methods forremoving CO₂ from a gas stream include using polar solvents to trap CO₂,but leave the other light gases in the gas stream. Such methods include:using methanol as a solvent (e.g., Rectisol® process); using di-methylethers of polyethylene glycol (e.g., Selexol™ process); using aminecomponents, such as, for example, monoethanlamine (MEA), diethanolamine(DEA), or methyl diethanolamine (MDEA), in water (e.g., Ucarsol™process); using potassium carbonate in water (e.g., Benfield™ process);and using caustic wash systems. However, in each of these systems CO₂ isthe main constituent of the product stream, and the CO₂ is removed fromthe product stream before other components are separated. The CO₂ thathas been removed is generally purged from the system in conventional CO₂separation systems.

With reference now to FIG. 1, a conventional CO₂ separation system 100,such as a system for using one of the above-described processes, will bedescribed. A reaction zone 110 converts a gas stream (not shown) into afeed stream 111 comprising light alkanes and CO₂. In embodiments, thefeed stream 111 also comprises one or more of CO, H₂, and methane. Thereactions that occur in the reaction zone 110 are not limited and may beany conventional reactions that form the desired light alkanes and CO₂as a byproduct. Such reactions include, for example, the conversion ofsyngas to light alkanes using a hybrid catalyst in a reactor. In someembodiments, the hybrid catalyst comprises a methanol synthesiscomponent and a solid microporous acid component. In other embodiments,different conventional reactions may be used to form light alkanes andCO₂ as a byproduct. It should be understood that, in embodiments, thereaction zone 110 may include any number of reactors. For instance, insome embodiments, the reaction zone 110 may comprise a first reactor forconverting raw gases—such as, for example, methane or natural gas—intosyngas, and the reaction zone 110 may comprise a second reactor—such as,for example, a reactor containing the above-described hybridcatalyst—for converting the syngas into light alkanes and a CO₂byproduct. Accordingly, in one or more embodiments, the reaction zone110 includes any necessary reactors for converting raw gas streams intofeed stream 111 that comprises light alkanes and CO₂.

The feed stream 111 is sent from the reaction zone 110 to a CO₂ scrubber120 that is fluidly connected to the reaction zone 110, a demethanizer140, and a stripper 130. The CO₂ scrubber 120 comprises a solvent thatisolates CO₂ from the other components of the feed stream 111—such as,for example, light alkanes and, optionally, one or more of CO, H₂, andCH₄. Any conventional solvent for isolating CO₂ may be used. Forexample, the solvent may comprise one or more of methanol, di-methylethers of polyethylene glycol, an aqueous solution comprising aminecomponents (such as, for example, MEA, DEA, or MDEA), or an aqueoussolution comprising potassium carbonate. Once the CO₂ has been isolatedfrom the other components of the feed stream 111, the CO₂ exits thescrubber 120 as CO₂ solvent stream 121 that comprises CO₂ and thesolvent described above. The CO₂ solvent stream 121 is sent to astripper 130 that is fluidly connected to the scrubber 120. Similarly,the other components of the feed stream 111 that have been isolated fromCO₂ (such as, for example, light alkanes and, optionally, one or more ofCO, H₂, and CH₄) exit the scrubber 120 as a first product stream 122.The first product stream 122 is sent from the scrubber 120 to ademethanizer 140. It should be understood that any conventional scrubbersuitable for scrubbing CO₂ from the feed stream 111 may be used as thescrubber 120.

The demethanizer 140 is fluidly connected to the scrubber 120 and thereaction zone 110. An optional first splitter 150 may be positionedbetween, and fluidly connected to, the demethanizer 140 and the reactionzone 110. At the demethanizer 140, the first product stream 122 isseparated into a final product stream 141 that comprises light alkanesand a recycle stream 142 that comprises one or more of H₂, CO, and CH₄.Any conventional type of demethanizer that is capable of separatinglight alkanes from other components in the first product stream 122 maybe used as the demethanizer 140. The final product stream 141 exits theconventional CO₂ separation system and may be used in various chemicalprocesses. The recycle stream 142 is sent from the demethanizer 140 tothe reaction zone 110 where the components of the recycle stream 142 canbe used as reactants in the reaction zone 110. In embodiments, thedemethanizer 140 is operated at a temperature of from −80° C. to −60°C., such as about −70° C., and at a pressure from 25 bar (2500 kPa) to35 bar (3500 kPa), such as about 30 bar (3000 kPa).

In some embodiments, the recycle stream 142 may comprise inert gases,such as, for example, nitrogen or argon, which, in some embodiments, maybe present in the feed stream 111. In such embodiments, an optionalfirst splitter 150 may be fluidly connected to the demethanizer 140 andthe first reaction zone 110 such that the recycle stream 142 passesthrough the first splitter 150. At the first splitter 150, a portion ofthe recycle stream 142 is removed from the conventional CO₂ separationsystem 100 as an inert gas containing stream 151. The remainder of therecycle stream 142 exits the first splitter 150 as a second recyclestream 152 and is sent to the reaction zone 110. In embodiments, aportion of the recycle stream 142 is withdrawn from the process toprevent inert build-up and the remaining portion of stream 142 is sentdirectly from the demethanizer 140 to the reaction zone 110. In one ormore embodiments, the recycle stream 142, the inert gas containingstream 151, and the second recycle stream 150 have the same composition.It should be understood that any conventional device that can separategas stream 142 into two streams and regulate the flow of gaseous stream142 in each of the two streams may be used as the first splitter 150.

As stated above, the CO₂ solvent stream 121 is sent from the scrubber120 to the stripper 130. The stripper 130 is fluidly connected to thescrubber 120 and a second splitter 160. At the stripper 130 the CO₂solvent stream 121 is stripped to form a lean solvent and gaseous CO₂.This stripping of the CO₂ solvent stream 121 can be conducted by anyconventional method, and is not limited herein. The solvent that remainsafter the CO₂ has been stripped therefrom exits the stripper 130 as asolvent stream 132 and is returned to the scrubber 120 where it canagain be used as a solvent to separate CO₂ from the feed stream 111.Similarly, the gaseous CO₂ that has been stripped from the CO₂ solventstream 121 exits the stripper 130 as CO₂ stream 131 and is sent to asecond splitter 160 that is fluidly connected to the stripper 130. Itshould be understood that any conventional stripper suitable forstripping CO₂ from the type of solvent used in the conventional CO₂separation system 100 may be used as the stripper 130. ConventionallyCO₂ separation from the solvent is achieved by adding energy to theprocess. This means adding heat or energy to the process stream. Athigher temperatures, part of the solvent may also evaporate, but it canbe recovered using condensation at low temperature. In embodiments,process heat, such as steam, and cooling, such as cooling water, areused for this process.

The second splitter 160 is fluidly connected to the stripper 130 and thereaction zone 110. At the second splitter 160 the gaseous CO₂ stream issplit into a CO₂ purge stream 161 that exits the conventional CO₂separation system 100 and a CO₂ recycle stream 162 that is sent back tothe reaction zone 110. It should be understood that the amount CO₂ thatis purged from the conventional CO₂ separation system 100 as CO₂ purgestream 161 and the amount of CO₂ that is sent back to the reaction zone110 is not limited and will be determined base on the need for CO₂ atthe reaction zone 110. It should be understood that any conventionaldevice that can separate gaseous CO₂ into two streams and regulate theflow of gaseous CO₂ in each of the two streams may be used as the secondsplitter 160.

The above method provides for recycling CO₂ (such as by CO₂ recyclestream 162) to be used in the reaction zone 110. However, there areinefficiencies with conventional CO₂ separation systems, such as the onedescribed above. One inefficiency is that a large amount of CO₂ must beremoved. For instance, in many systems the mass ratio of CO₂ to alkaneat the outlet of the reaction zone 110 is greater than one. When the CO₂to alkane ratio is greater than one, more than one pound of CO₂ must beremoved for every pound of alkanes, which requires a large amount ofenergy per pound of alkane produced. Another inefficiency of theconventional CO₂ separation systems, such as those described above, isthat the CO₂ recycle stream 162 that exits the stripper 130 and is sentback to the reaction zone 110 is at a low pressure, so it needs to becompressed before it can be used in the reaction zone 110, whichrequires additional capital investment and energy.

In view of the above inefficiencies of conventional CO₂ separationsystems, it is desirable to separate H₂, CO, CO₂, and CH₄ into onestream and light alkanes into another stream. This separation scheme isnot easily achieved because ethane (i.e., C₂ alkane) and CO₂ have anazeotrope and cannot be separated by simple distillation. However,systems and methods for separating CO₂ during the preparation of alkanesaccording to embodiments disclosed and described below can achieve thispreferred separation scheme.

With reference now to FIG. 2, systems and methods for separating CO₂during alkane preparation using a two column distillation according toone or more embodiments is described. In the embodiments of CO₂separation systems 200 shown in FIG. 2, a small amount of CO₂ (i.e., apurge amount of CO₂) is removed from the feed stream 111 in a CO₂separator 210 before the process stream 212 comprising light alkanes isintroduced into a first separation zone 230. However, unlike theconventional CO₂ separation systems described in reference to FIG. 1above, in the CO₂ separation system 200 according to embodiments shownin FIG. 2, the process stream 212 that enters the first separation zone230 comprises a significant amount of CO₂. In one or more embodiments,the process stream 212 comprises from 5 mass % to 40 mass % CO₂, such asfrom 10 mass % to 35 mass % CO₂, from 15 mass % to 30 mass % CO₂, orfrom 20 mass % to 25 mass % CO₂. In the embodiments of CO₂ separationsystems depicted in FIG. 2, CO₂ is primarily separated from the lightalkanes, including ethane, in the first separation zone 230 using ahydrocarbon solvent. Details of the CO₂ separation systems 200 andmethods according to embodiments depicted in FIG. 2 are described below.

In one or more embodiments, the CO₂ separation system 200 comprises areaction zone 110 that is the same as the reaction zone 110 describedabove in reference to the conventional CO₂ separation systems asdiscussed above. A feed stream 111 that comprises light alkanes, CO₂ andone or more of H₂, CO, and CH₄ is sent from the reaction zone 110 to aCO₂ separator 210 that is fluidly connected to the reaction zone 110 anda CO₂ stripper 220. According to embodiments, in the CO₂ separator 210,the feed stream 111 is mixed with an amine solvent, such as, for exampleMEA, DEA, MDEA, or mixtures thereof, that isolates a small amount of CO₂from the remaining components of the feed stream 111, such as, forexample, light alkanes, CO, H₂, and CH₄. The amount of amine solvent andreaction conditions in the CO₂ separator 210 are selected, in variousembodiments, such that only a small amount of CO₂ is isolated in the CO₂separator 210.

The amount of CO₂ that is isolated by the amine solvent is, in one ormore embodiments, an amount of CO₂ that is desired to be purged from theCO₂ separation system 200. The desired amount of CO₂ that is desired tobe purged from the CO₂ separation system 200 is, in some embodiments,based on the amount of CO₂ that is to be recycled back to the reactionzone 110. Although not limited to any particular theory, the amount ofCO₂ co-produced with light alkanes in the reaction zone 110 may dependon the combination of reactors and processes used in the reaction zone110. It should be understood that it may also depend on the H₂:CO molarratio used in the synthesis of light alkanes in reaction zone 110. Inone or more embodiments, the molar H₂:CO ratio is from 1:1 to 10:1 suchas from 7:1 to 9:1, or about 8:1. In some embodiments, the molar H₂:COratio is from 3:1 to 5:1, or about 3:1. In embodiments, a CO₂ solventstream 211 comprising the purge amount of CO₂ and the amine solventexits the CO₂ separator 210 and is sent to the CO₂ stripper 220. At theCO₂ stripper 220, the CO₂ in the CO₂ solvent stream 211 is extractedfrom the amine solvent and purged from the CO₂ separation system 200 asCO₂ purge 221. In various embodiments, after the CO₂ has been extractedfrom the CO₂ solvent stream 211, the amine solvent is sent from the CO₂stripper 220 to the CO₂ separator 210 as solvent stream 222. It shouldbe understood that in one or more embodiments, the CO₂ stripper 220 isany conventional extractor that is capable of extracting CO₂ from anamine solvent.

As discussed above, according to one or more embodiments, a processstream 212 that comprises light alkanes, CO₂, and one or more of CO, H₂,and CH₄ is sent from the CO₂ separator 210 to the first separation zone230. The first separation zone 230 is fluidly connected to the CO₂separator 210, the reaction zone 110, and a second separation zone 240.In the first separation zone 230, according to various embodiments, thelight alkanes in the process stream 212 are separated from CO₂ and oneor more of CO, H₂, and CH₄ that are present in the process stream 212.In some embodiments, this separation may be conducted by any suitableprocess. However, in one or more embodiments, the first separation zone230 is a combined demethanizer/extractive distillation column thatseparates light alkanes from CO₂ and one or more of CO, H₂, and CH₄. Inone or more embodiments, the separation zone 230 comprises a hydrocarbonsolvent for separating the light alkanes from CO₂ and one or more of CO,H₂, and CH₄. In embodiments, the hydrocarbon solvent may be C₃ to C₅alkanes that are recycled from the second separator 240 as described inmore detail below. A light alkane stream 231 that comprises C₂ to C₅alkanes exits the first separation zone 230 and is sent to the secondseparation zone 240. A recycle stream 232 comprising CO₂ and one or moreof CO, H₂, and CH₄ exits the first separation zone 230 and is sent backto the reaction zone 110.

In some embodiments, the recycle stream 232 may comprise inert gases,such as, for example, nitrogen or argon, which, in some embodiments, maybe present in the feed stream 111. In such embodiments, an optionalfirst splitter 260 may be fluidly connected to the first separation zone230 and the reaction zone 110 such that the recycle stream 232 passesthrough the first splitter 260. At the first splitter 260, a portion ofthe recycle stream 232 is removed from the CO₂ separation system 200 asan inert gas containing stream 261. The remainder of the recycle stream232 exits the first splitter 260 as a second recycle stream 262 and issent to the reaction zone 110. In embodiments, a portion of the recyclestream 232 is withdrawn from the process to prevent inert build-up andthe remaining portion of stream 232 is sent directly from the firstseparation zone 230 to the reaction zone 110 as the second recyclestream 262. In one or more embodiments, the recycle stream 232, theinert gas containing stream 261, and the second recycle stream 262 havethe same composition. It should be understood that any conventionaldevice that can separate gas stream 232 into two streams and regulatethe flow of gaseous stream 232 in each of the two streams may be used asthe first splitter 260.

As discussed above, in embodiments, a light alkane stream 231 exits thefirst separation zone 230 and is sent to the second separation zone 240.The second separation zone 240 is, in embodiments, fluidly connected tothe first separation zone 230 and a second splitter 250. In the secondseparation zone 240, the light alkanes are separated into a firstproduct stream 241 comprising C₂ alkanes and a second product stream 242that comprises C₃ to C₅ alkanes. In some embodiments, the first productstream 241 comprises from 10 mass % to 90 mass % C₂ alkanes, such asfrom 20 mass % to 80 mass % C₂ alkanes, from 30 mass % to 70 mass % C₂alkanes, or from 30 mass % to 60 mass % C₂ alkanes. In one or moreembodiments, the first product stream 241 consists essentially of C₂ toC₃ alkanes. This separation of the light alkanes into the first productstream 241 that comprises C₂ alkanes and the second product stream 242that comprises C₃ to C₅ alkanes may, in various embodiments, becompleted by any known separation method, such as, for exampledistillation. In one or more embodiments, the second product stream 242comprises from 30 mass % to 95 mass % C₃ to C₅ alkanes, such as from 40mass % to 90 mass % C₃ to C₅ alkanes, from 50 mass % to 90 mass % C₃ toC₅ alkanes, or from 60 mass % to 85 mass % C₃ to C₅ alkanes. The firstproduct stream 241 exits the CO₂ separation system 200 and can be usedas products or starting materials in other chemical processing. In someembodiments, the second product stream 242 exits the second separationzone 240 and is sent to the second splitter 250 that is fluidlyconnected to the second separation zone 240 and the first separationzone 230.

According to one or more embodiments, the second product stream 242 issplit at the second splitter 250 into a third product stream 251 and ahydrocarbon solvent stream 252. In embodiments, the second productstream 242 is physically split into the third product stream 251 and thehydrocarbon solvent stream 252 and, thus, the third product stream 251has the same composition as the hydrocarbon solvent stream 252. In oneor more embodiments, the third product stream 251 exits the CO₂separation system 200 and can be used as products or starting materialsin other chemical processing. The hydrocarbon solvent stream 252, whichcomprises C₃ to C₅ alkanes, is sent back to the first separation zone230, where, in one or more embodiments, it is used as a solvent toseparate the process stream 212 into the recycle stream 232—thatcomprises CO₂ and one or more of CO, H₂, and CH₄—and light alkane stream231. It should be understood that, in embodiments, any splitter capableof separating the second product stream 242 into two streams may be usedas the second splitter 250.

As discussed above, in some embodiments, the hydrocarbon solvent stream252 exits the second splitter 250 and is sent to the first separationzone 230 where it is used as a solvent to separate process stream 212into light alkane stream 231 and recycle stream 232. In embodiments, theamount of hydrocarbon solvent 252 that is directed to the firstseparation zone 230 is an amount such that the weight ratio ofhydrocarbon solvent in the first separation zone 230 to the lightalkanes in the first separation zone 230 is from 1:1 to 5:1, such asfrom 1:1 to 3:1, or from 2:1 to 3:1.

With reference now to FIG. 3, further embodiments of systems and methodsfor separating CO₂ during alkane preparation using a two columndistillation are described. In the embodiments of CO₂ separation systems300 shown in FIG. 3, the feed stream 111 is fed directly to the firstseparation zone 230 without removing any CO₂ from the feed stream 111.Details of the CO₂ separation systems 300 and methods according toembodiments depicted in FIG. 3 are described below.

In one or more embodiments, the CO₂ separation system 300 comprises areaction zone 110 that is the same as the reaction zone 110 describedabove in reference to the conventional CO₂ separation systems depictedin FIG. 1 and the CO₂ separation systems depicted in FIG. 2. In someembodiments, a feed stream 111 that comprises light alkanes, CO₂ and oneor more of H₂, CO, and CH₄ is sent from the reaction zone 110 to a firstseparation zone 230. The first separation zone 230 is fluidly connectedto the reaction zone 110 and a second separation zone 240. In the firstseparation zone 230, according to various embodiments, the light alkanesin the feed stream 111 are separated from the CO₂ and one or more of CO,H₂, and CH₄ that are present in the feed stream 111. In embodiments,this separation may be conducted by any suitable process. However, inone or more embodiments, the first separation zone 230 is a combineddemethanizer/extractive distillation column that separates light alkanesfrom CO₂ and one or more of CO, H₂, and CH₄. In some embodiments, thefirst separation zone 230 comprises a hydrocarbon solvent for separatingthe light alkanes from CO₂ and one or more of CO, H₂, and CH₄. Inembodiments, the hydrocarbon solvent may be C₃ to C₅ alkanes that arerecycled from the second separation zone 240 as described in more detailbelow. In various embodiments, a second process stream 233 thatcomprises C₂ to C₅ alkanes and a small amount of CO₂ exits the firstseparation zone 230 and is sent to the second separation zone 240. Arecycle stream 232 comprising CO₂ and one or more of CO, H₂, and CH₄exits the first separation zone 230 and is sent back to the reactionzone 110.

In some embodiments, the recycle stream 232 may comprise inert gases,such as, for example, nitrogen or argon, which may be introduced by thefeed stream 111. In such embodiments, an optional first splitter 260 maybe fluidly connected to the first separation zone 230 and the reactionzone 110 such that the recycle stream 232 passes through the firstsplitter 260. At the first splitter 260, a portion of the recycle stream232 is removed from the CO₂ separation system 300 as an inert gascontaining stream 261. The remainder of the recycle stream 232 exits thefirst splitter 260 as a second recycle stream 262 and is sent to thereaction zone 110. In embodiments, a portion of the recycle stream 232is withdrawn from the process to prevent inert build-up and theremaining portion of the recycle stream 232 is sent directly from thefirst separation zone 230 to the reaction zone 110 as the second recyclestream 262. In one or more embodiments, the recycle stream 232, theinert gas containing stream 261, and the second recycle stream 262 havethe same composition. It should be understood that any conventionaldevice that can separate the recycle stream 232 into two streams andregulate the flow of the recycle stream 232 in each of the two streamsmay be used as the first splitter 260.

As stated above, in embodiments, a second process stream 233 exits thefirst separation zone 230 and is sent to the second separation zone 240.The second separation zone 240 is, in embodiments, fluidly connected tothe first separation zone 230 and a second splitter 250. In the secondseparation zone 240, the light alkanes in the second process stream 233are separated into a third process stream 243—that comprises C₂ alkanesand a small amount of CO₂ (i.e., a purge amount of CO₂)—and a secondproduct stream 242 that comprises C₃ to C₅ alkanes. This separation ofthe light alkanes into the third process stream 243 and the secondproduct stream 242 may, in various embodiments, be completed by anyknown separation method, such as, for example distillation. The thirdprocess stream 243 exits the second separation zone 240 and is sent tothe CO₂ separator 210. The second product stream 242 exits the secondseparation zone 240 and is sent to the second splitter 250 that isfluidly connected to the second separation zone 240 and the firstseparation zone 230.

According to embodiments, at the second splitter 250 the second productstream 242 is split into a third product stream 251 and a hydrocarbonsolvent stream 252. In one or more embodiments, the second productstream 242 comprises from 30 mass % to 95 mass % C₃ to C₅ alkanes, suchas from 40 mass % to 90 mass % C₃ to C₅ alkanes, from 50 mass % to 90mass % C₃ to C₅ alkanes, or from 60 mass % to 85 mass % C₃ to C₅alkanes. In embodiments, the second product stream 242 is physicallysplit into the third product stream 251 and the hydrocarbon solventstream 252 and, thus, the third product stream 251 has the samecomposition as the hydrocarbon solvent stream 252. The third productstream 251 exits the CO₂ separation system 300 and can be used asproducts or starting materials in other chemical processing. Thehydrocarbon solvent stream 252, which comprises C₃ to C₅ alkanes, issent back to the first separation zone 230, where, in one or moreembodiments, it is used as a hydrocarbon solvent to separate the feedstream 111 into the recycle stream 232—that comprises CO₂ and one ormore of CO, H₂, and CH₄—and second process stream 233. It should beunderstood that, in embodiments, any splitter capable of separating thesecond product stream 242 into two streams may be used as the secondsplitter 250.

As discussed above, in some embodiments, the hydrocarbon solvent stream252 exits the second splitter 250 and is sent to the first separationzone 230 where it is used as a solvent to separate feed stream 111 intothe second process stream 233 and recycle stream 232. In embodiments,the amount of hydrocarbon solvent 252 that is directed to the firstseparation zone 230 is an amount so that the weight ratio of hydrocarbonsolvent in the first separation zone 230 to the amount of light alkanesin the first separation zone 230 is from 1:1 to 5:1, such as from 1:1 to3:1, or from 2:1 to 3:1.

As discussed above, in one or more embodiments, the third process stream243 exits the second separation zone 240 and is sent to the CO₂separator 210 that is fluidly connected to the second separation zone240 and a CO₂ stripper 220. In one or more embodiments, the thirdprocess stream 243 comprises from 5 mass % to 40 mass % CO₂, such asfrom 10 mass % to 35 mass % CO₂, from 15 mass % to 30 mass % CO₂, orfrom 20 mass % to 25 mass % CO₂. According to embodiments, in the CO₂separator 210, the third process stream 243 is mixed with an aminesolvent, such as, for example MEA, DEA, MDEA, or mixtures thereof, thatisolates the small amount of CO₂ (i.e., the purge amount of CO₂)remaining in the third process stream 243. The amount of amine solventand reaction conditions in the CO₂ separator 210 are selected, invarious embodiments, such that only the small amount of CO₂ is isolatedin the CO₂ separator 210. As described above, the desired amount of CO₂that is to be purged from the CO₂ separation system 300 is, in someembodiments, based upon the amount of CO₂ that is to be recycled back tothe reaction zone 110. Namely, in embodiments, the amount of CO₂ that isto be recycled back to the reaction zone 110 is included in recyclestream 232. Thus, any difference between the amount of CO₂ in the feedstream 111 and the desired amount that is included in the recycle stream232 is sent to the CO₂ separator 210 to be isolated by the aminesolution and ultimately purged from the CO₂ separation system.

In embodiments, a CO₂ solvent stream 211 comprising the purge amount ofCO₂ and the amine solvent exits the CO₂ separator 210 and is sent to theCO₂ stripper 220. At the CO₂ stripper 220, the CO₂ in the CO₂ solventstream 211 is stripped from the amine solvent and purged from the CO₂separation system 300 as CO₂ purge 221. In various embodiments, afterthe CO₂ has been stripped from the CO₂ solvent stream 211, the aminesolvent is sent from the CO₂ stripper 220 to the CO₂ separator 210 assolvent stream 222. It should be understood that in one or moreembodiments, the CO₂ stripper 220 is any conventional stripper that iscapable of stripping CO₂ from an amine solvent.

According to one or more embodiments, a fourth product stream 213 thatcomprises C₂ alkanes exits the CO₂ separator 210 and the CO₂ separationsystem 300 where it can be used as a product or starting materials forvarious chemical processes. In some embodiments, the fourth productstream 213 comprises from 10 mass % to 90 mass % C₂ alkanes, such asfrom 20 mass % to 80 mass % C₂ alkanes, from 30 mass % to 70 mass % C₂alkanes, or from 30 mass % to 60 mass % C₂ alkanes. In one or moreembodiments, the fourth product stream 213 consists essentially of C₂ toC₃ alkanes.

The systems and method for separating CO₂ in the preparation of alkanesaccording to embodiments disclosed and described herein reduce theenergy required to separate CO₂ from the alkane-containing productstream. Because only a small amount of CO₂ is absorbed into the solventto isolate the CO₂ from the light alkanes, only a small fraction of theenergy required in conventional CO₂ separation systems. Further, therecycle stream, which comprises CO₂, described in embodiments herein ispressurized, thus no, or very little, compression of the recycle streamis required before it is introduced into the reaction zone 110.

EXAMPLES

Embodiments will be further clarified by the following examples, whichwere simulated using Aspen simulation software.

Example 1

A gas feed containing H₂, CH₄, CO, CO₂, ethane, propane, butane, andpentane was separated into three streams using two columns. A portion ofCO₂, which must be removed from the system, was separated before a firstseparation zone. For this example, the CO2 purge rate was 16,400 kg/hr.The first column was a distillation column with a solvent feed on thetop tray. The overhead gas stream product for recycle back to thereactor contained H₂, CO, CO₂, and CH₄. The remaining products wereseparated into two streams by distillation. The overhead product streamcontained C₂ and some C₃. A portion of the tails stream was used as thesolvent to the first column, and the remainder was the tails productcontaining C₃, C₄, and C₅. The specifics of the distillation columns areprovided in Table 1:

TABLE 1 Column 1 (Extractive Distillation) Number of Trays 52 Feed Tray35 Solvent Feed Tray 1 Column 2 Number of Trays 35 Feed Tray 10

The reflux rates and heat loads on column 1 and column 2 are provided inTable 2:

TABLE 2 Column 1 Reflux Rate 186,284 kg/hr Q_(condenser1) −32.6 MMBtu/hrQ_(reboiler1) 46.7 MMBtu/hr Column 2 Reflux Rate 80,000 kg/hrQ_(condenser2) −28.4 MMBtu/hr Q_(reboiler2) 27.0 MMBtu/hr

Table 3 below provides mass balance for all the streams of Example 1.The streams described in Table 3 are as follows: D1 is the overhead flowfrom column 1; B1 is the bottoms flow from column 1 and the feed tocolumn 2; D2 is the overhead flow from column 2; B2 is the bottoms flowfrom column 2; B2 product is the portion of B2 taken out as product; B2recycle is the solvent feed to column 1. The total alkane productionrate was about 36,600 kg/hr.

TABLE 3 Mass Balance for all Streams for Example 1 Solvent Gas Feed (B2B2 Feed recycle) D1 B1 D2 B2 product Temp, ° C. 30.9 −11.4 −39.6 98.631.6 109.8 109.8 P, bar_(g) 33.5 33.5 33.5 33.5 27.6 27.6 27.6 flow,kg/hr 114,359 100,000 77,773 136,586 23,658 112,927 12,927 Total Flow5692 1854 4807 2739 645 2094 240 kmol/hr Composition in mole fraction H₂0.4568 0.0000 0.5408 0.0000 0.0000 0.0000 0.0000 CO 0.0469 0.0000 0.05550.0000 0.0000 0.0000 0.0000 CO₂ 0.1964 0.0000 0.2312 0.0024 0.01040.0000 0.0000 CH₄ 0.1275 0.0000 0.1510 0.0000 0.0000 0.0000 0.0000 C₂H₆0.0630 0.0018 0.0032 0.1265 0.5311 0.0018 0.0018 C₃H₈ 0.0842 0.45310.0158 0.4539 0.4564 0.4531 0.4531 C₄H₁₀₋₀₁ 0.0184 0.3866 0.0022 0.29600.0021 0.3866 0.3866 C₅H₁₂₋₀₁ 0.0068 0.1585 0.0002 0.1212 0.0000 0.15850.1585

The energy requirement for the separation can be calculated on a fuelgas equivalent basis. For this comparison, energy for steam andelectrical generation must be put on a consistent basis. The efficiencyfor converting fuel gas to steam is selected to be at 85%. The coolingrequirement must be converted to the electrical power needed to performthe cooling, which depends on the cooling temperature. A relationshipbetween cooling temperature and electrical power is taken from Hall,“Rules of Thumb for Chemical Engineers”, p. 194, Chapter 11. Selectedvalues are given in Table 4 below.

TABLE 4 HP/Ton T, ° C. Refrigeration −17.8 1.75 −40.0 3.01 −51.1 3.79−73.3 5.69 −95.6 8.18In addition, the electrical power must be converted to a fuel gasequivalent. For this analysis, the efficiency for converting fuel gas toelectrical energy is selected as 34%.

Energy Requirement

Cooling Power Fuel Gas Cooling Temp, Duty Requirement Equivalent, ° C.MMBtu/hr kw/(MMBtu/hr) MMBtu/yr −39.6 C. −32.6 187 61For CO2 removal, the energy requirement basis is assumed to be 2 GJ/tonCO₂, or 860 Btu/lb. This is taken from Straelen, and Geuzebroek, “TheThermodynamic minimum regeneration energy required for post-combustionCO₂ capture”, ScienceDirect, 2010. The energy breakdown in terms of fuelgas equivalent in given in Table 5 below.

TABLE 5 Fuel Gas Equivalent Energy in MMBtu/hr Refrigeration 61 CO₂Removal 31 Reboiler 1 54 Reboiler 2 32 Total Energy 178 Unit Energy,Btu/lb alkane 2240 Btu/lb alkane product

Example 2

In this example, the portion of CO₂ that was removed from the reactionloop was included in the feed to the first distillation column. This CO₂leaves the first column in the tails with the other C₂₊ alkanecomponents, and ends up in the second column product.

The reflux rates and heat loads on column 1 and column 2 are provided inTable 6:

TABLE 6 Column 1 Reflux Rate 177,294 kg/hr Q_(condenser1) −29.7 MMBtu/hrQ_(reboiler1) 27.8 MMBtu/hr Column 2 Reflux Rate 42,000 kg/hrQ_(condenser2) −23.0 MMBtu/hr Q_(reboiler2) 31.8 MMBtu/hr

The mass balance for all streams is given in Table 7. The streams inTable 7 have the same designations as the streams in Table 3 of Example1.

TABLE 7 Mass Balance for all Streams for Example 2 Gas Solvent B2 FeedFeed D1 B1 D2 B2 product Temp, ° C. 30.5 −10.5 −38.0 64.0 0.6 109.1109.1 P, bar_(g) 33.5 33.5 33.5 33.5 27.6 27.6 27.6 flow, kg/hr 131,236100,000 78,534 152,703 39,604 113,082 13,082 Total Flow 6075 1860 48243111 1008 2103 243 kmol/hr mole fraction H₂ 0.4280 0.0000 0.5390 0.00000.0000 0.0000 0.0000 CO 0.0439 0.0000 0.0553 0.0000 0.0000 0.0000 0.0000CO₂ 0.2454 0.0000 0.2318 0.1198 0.3696 0.0000 0.0000 CH₄ 0.1195 0.00000.1504 0.0002 0.0005 0.0000 0.0000 C₂H₆ 0.0590 0.0018 0.0032 0.11130.3400 0.0018 0.0018 C₃H₈ 0.0803 0.4629 0.0176 0.4063 0.2882 0.46290.4629 C₄H₁₀₋₀₁ 0.0174 0.3791 0.0024 0.2568 0.0017 0.3791 0.3791C₅H₁₂₋₀₁ 0.0064 0.1562 0.0002 0.1056 0.0000 0.1562 0.1562

Table 8 below provides the energy requirements for Example 2, which werecalculated in the same manner as provided above in Example 1.

TABLE 8 Cooling Power Fuel Gas Cooling Temp, Duty RequirementEquivalent, C. MMBtu/hr kw/(MMBtu/hr) MMBtu/yr −38 C. −29.7 180 54 0.−22.9 62 14

For CO₂ removal, the energy requirement basis is assumed to be 860Btu/lb CO₂. The energy breakdown in terms of fuel gas equivalent ingiven in Table 9 below.

TABLE 9 Fuel Gas Equivalent Energy in MMBtu/hr Refrigeration 68 CO₂Removal 31 Reboiler 1 33 Reboiler 2 37 Total Energy 169 Unit Energy,Btu/lb alkane 2116 Btu/lb alkane product

This case gives a slightly higher refrigeration cost due to the loweroverhead temperature of column D2, but reduced reboiler cost for thefirst column. The net result is slightly lower energy cost/lb alkaneproduct.

COMPARATIVE EXAMPLE

In this Comparative Example, a conventional separation system, such asthe system shown in FIG. 1 was used. For this Comparative Example, allCO₂, (i.e., 65,600 kg/hr), was removed from the feed gas. A portion ofthe CO₂, 16,400 kg/hr, was purged from the process, and the remainderwas compressed and recycled back to the reactor. The remaining gasstream after the CO₂ removal was cooled in steps to −100° C. Thecondensed liquid was fed to a demethanizer distillation column, whichseparated the C₂₊ alkanes in the tails, and the overhead contained CH₄,CO, and some H₂. The design specifications on the column were 0.0001mass purity CH₄ in the tails and 0.005 mass purity ethane in theoverhead, which were met by controlling the reflux ratio and thedistillate to feed (D/F) ratio.

The uncondensed gas feed contained mostly H₂ (73%), CH₄ (18%), CO(7.3%), and C₂H₆ (1.9%). The ethane concentration was reduced further byexpanding this stream through a turboexpander for cooling, and feedingthe condensed product back to the column. The cold gas stream was usedto cool the feed. The gas stream, containing H₂, CH₄, and CO wascompressed back to reactor pressure for recycle, and the overall recyclecomposition was the same as in Example 1.

Results of the energy balance are given below in Table 10 below:

TABLE 10 Fuel Gas Equivalent Energy in MMBtu/hr Refrigeration 38 CO₂Removal 124 CO₂ Compression 42 Recycle Gas Compression 40 ColumnReboiler 14 Total Energy 259 Unit Energy, Btu/lb alkane 3252 Btu/lbalkane product

This Comparative Example shows the increased energy usage of about 53%used in a conventional CO₂ removal system compared to the CO₂ removalsystem of Example 1 and 45% more energy used in the conventional CO₂removal system compared to the CO₂ removal system of Example 2. Inaddition, the conventional CO₂ removal system was required to be about 4times bigger due to a 4 times higher CO₂ removal rate. This conventionalapproach also requires compression of both the recycled CO₂ and therecycled H₂-rich stream, which requires additional capital.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the embodiments describedherein without departing from the spirit and scope of the claimedsubject matter. Thus it is intended that the specification cover themodifications and variations of the various embodiments described hereinprovided such modification and variations come within the scope of theappended claims and their equivalents.

The invention claimed is:
 1. A method for separating CO₂ from C₂ to C₅alkanes, comprising: introducing a first stream comprising C₂ to C₅alkanes and CO₂ into a first separation zone, the first separation zonecomprising a hydrocarbon solvent; separating the first stream into arecycle stream and a second stream in the first separation zone, whereinthe recycle stream comprises CO₂ and one or more of CO, H₂, and CH₄, andthe second stream comprises C₂ to C₅ alkanes; introducing the recyclestream into a reaction zone; introducing the second stream into a secondseparation zone; and separating the second stream into a third streamand a fourth stream, wherein the third stream comprises C₂ alkanes andthe fourth stream comprises C₃ to C₅ alkanes.
 2. The method of claim 1,wherein the method further comprises introducing at least a portion ofthe fourth stream into the first separation zone as the hydrocarbonsolvent.
 3. The method of claim 1, wherein a portion of the recyclestream is purged before the recycle stream is introduced into thereaction zone.
 4. The method of claim 1, wherein a ratio of thehydrocarbon solvent to C₂ to C₅ alkanes in the first separation zone isfrom 1:1 to 5:1.
 5. The method of claim 1, wherein the first stream is afeed stream that is introduced from a reaction zone into the firstseparation zone.
 6. The method of claim 1, wherein the third streamfurther comprises CO₂, and the method further comprises introducing thethird stream to a CO₂ separator where a purge amount of CO₂ separatedfrom the third stream.
 7. The method of claim 1, wherein the methodfurther comprises removing a purge amount of CO₂ from the first streambefore it is introduced into the first separation zone.
 8. The method ofclaim 6, wherein removing the purge amount of CO₂ comprises introducinga stream comprising CO₂ into a CO₂ separator that comprises an aminesolvent, and isolating the purge amount of CO₂ in the amine solvent. 9.The method of claim 8, wherein the method further comprises: introducingthe amine solvent and the purge amount of CO₂ into a CO₂ stripper thatstrips the purge amount CO₂ from the amine solvent thereby forming astripped amine solvent, and introducing the stripped amine solvent intothe CO₂ separator.
 10. The method of claim 1, wherein the fourth streamcomprises C₃ alkanes and C₄ to C₅ alkanes.
 11. The method of claim 1,wherein the recycle stream comprises CH₄.